The present invention relates to sintering of core-shell metal, semiconductor, and ceramic powders to form structures and porous films.
With the European Union and Chinese bans of Lead-Tin (Pb—Sn) eutectic solder for electronic interconnects, the microelectronics research community has been examining a range of possible Pb-free alternatives for interconnection, even non-solder based technologies. The dominant Pb-free solders currently used in high-volume consumer electronics are near eutectic Tin-Silver-Copper (Sn—Ag—Cu or SAC) alloys, with some use of Sn—Cu alloys with ternary additions to modify wetting and interactions with substrate materials. Although consumer electronics have transitioned almost entirely to Pb-free solder interconnects, their poorer drop/impact behavior compared with Sn—Pb eutectic, their higher processing temperatures (240° C.) relative to Sn—Pb eutectic (220° C.), their highly anisotropic solidification, thermal expansion, and mechanical behavior, and their propensity to spontaneously form tin whiskers have left the microelectronics industry looking for improved interconnect solutions.
The electronics industry is searching for alternatives to tin-lead and lead-free solder alloys for board level interconnection, flip-chip and other area array interconnections, thermal interface materials, and die attach materials. Sintering of Cu—Ag core shell particles is an exemplary embodiment disclosed herein for these applications. The sintered Cu—Ag does not form tin whiskers, a reliability risk for lead-free solders, because the interconnect material Cu—Ag does not contain tin. The Cu—Ag core-shell particles are more mechanically and chemically compatible than solder with Cu board pads and traces and with metal leads and lead-less interconnects on the components.
The printed electronics industry is searching for a method to create more dimensionally stable, more corrosion resistant, and cheaper metallic interconnections than pure Ag nanoparticles. The printed electronics industry is also searching for a method to create sintered semiconductor layers at temperatures far below those required to sinter semiconductor particles without the shell. Embodiments disclosed herein include a breakup of the metal shell into isolated metal particles which produces structures that behave electrically as a semiconductor and not a metal. This approach can work for amorphous semiconductor powders as well as crystalline powders.
Embodiments disclosed herein can also benefit the sensor industry by allowing for the joining of higher temperature sensor materials with fast diffusing shell materials. Embodiments disclosed herein can also benefit other industries which require assembly of particles to form continuous structures at temperatures where the core particles do not sinter.
A solderless nanotechnology based on low temperature sintering is disclosed with exemplary embodiments of Cu—Ag core-shell nanoparticles to form porous interconnections that have the potential to replace traditional solder joints as well as high-Pb and Au-containing solder alloys used for high temperature die attach of semiconductor devices. For Cu—Ag core-shell particles, Ag diffusion from the particle surfaces to particle-particle contacts during heating leads to enhanced interparticle sintering compared with uncoated Cu nanoparticles. Microstructural, thermal and electrical characteristics of the sintered structures indicate that Cu—Ag core-shell sintering may be a viable route to a solderless alternative to Pb-free solders with a joint formation temperature less than or equal to Sn—Pb eutectic.
Enhanced sintering of Cu—Ag core-shell nanoparticles was observed to occur by fast diffusion of Ag at 220° C. from particle surfaces, leading to the formation of sintered necks of Ag at the initial particle-particle contacts. In comparison with similar sized pure Cu nanoparticles after annealing and the Cu—Ag nanoparticles before annealing, Cu—Ag particles had higher densities. Transmission electron microscopy (TEM) and energy-filtered TEM shows that the sintered necks were primarily or pure Ag and that the particle surfaces away from the contacts were nearly Ag-free, in contrast to the uniform shell thickness of the as-synthesized Cu—Ag core-shell nanoparticles. The extent of neck formation in the final sintered structure can be controlled by the choice of the initial Ag layer thickness.
A method is disclosed for forming a sintered structure at an annealing temperature. The method includes obtaining core-shell particles having a core material and a shell material, forming the core-shell particles into a powder compact in which the core-shell particles are in physical contact, and annealing the powder compact at the annealing temperature. The shell material is a metal material that diffuses faster than the core material at the annealing temperature and the faster diffusing shell material diffuses to the contacts between the core-shell particles during the annealing to form sintered interfaces between the core-shell particles that are in physical contact. The core material can be a metal, semiconductor or ceramic material. The core material can be copper and/or the shell material can be silver. The sintered interfaces can be almost purely composed of the shell material.
The core material and the shell material can have limited mutual solubility at the annealing temperature. The core material and the shell material can be materials that do not form an intermediate phase between the core and shell materials at the annealing temperature. The annealing temperature can be significantly lower than the temperature needed to form sintered interfaces between particles of the core material without the shell material. The annealing step can be performed in an annealing atmosphere that promotes diffusion of the shell material at the annealing temperature, where the shell material may not be prone to diffuse at the annealing temperature in a standard air atmosphere. The annealing step can be performed long enough to form discontinuous regions of the shell material in the sintered structure between the sintered interfaces. The annealing step can cause an increase in the density of the core-shell particles in the powder compact. The core-shell particles can have an average diameter of approximately 470 nm. The average thickness of the shell material on the core-shell particles can be approximately 7 nm.
A sintered structure is disclosed that includes a sintered core-shell compact made of a plurality of core-shell particles sintered at an annealing temperature, where each of the plurality of core-shell particles has a core material and a shell material, the shell material being a metal material that diffuses faster than the core material at the annealing temperature. The sintered core-shell compact includes a plurality of sintered interfaces between the core-shell particles that are in physical contact. The core material can be copper and/or the shell material can be silver. The core material can be a metal material, a semiconductor material, or a ceramic material. The sintered core-shell compact can be formed so it does not include an intermediate phase material between the core and shell materials. The sintered interfaces may be almost purely composed of the shell material. The sintered core-shell compact can include discontinuous regions of the shell material on the sintered core-shell particles between the sintered interfaces.